The present disclosure relates to methods and devices that are useful for adjusting the optical power of a lens, particularly an intraocular lens that can be implanted in the eye.
The lens of the eye is used to focus light on the retina. The lens is usually clear, but can become opaque (i.e. develop a cataract) due to age or certain diseases. The usual treatment is to surgically remove the opaque lens and replace it with an artificial or intraocular lens.
It is desirable to be able to adjust the lens after it has been implanted. Such adjustments are useful in correcting misplacement of the lens during the surgical operation and/or to treat higher order optical aberrations. A common method is to use ultraviolet (UV) activation to induce the change in lens performance, to allow for high spatial resolution of the adjustment (due to the low wavelength of UV). After the lens is adjusted, the lens should not appreciably change in performance over the lifetime of the lens.
U.S. Pat. No. 7,134,755 describes a lens that uses ultraviolet light curable monomers in a silicone polymer matrix. The monomers are selectively polymerized using a digital light delivery system to alter the lens power at specific points.
There are two distinct effects that alter the lens optical power in this system. First, the polymerization of the UV curable monomers changes the refractive index of the system from n=1.4144 to n=1.4229, which would increase the optical power of the test lens from 95.1 diopters to 96.7 diopters. This change in the lens power is much smaller than the change in lens power that was reported in the patent, indicating this is not the primary mechanism of index change in this patent.
The second effect, which is responsible for the largest component of the change in lens optical power, is a swelling of the lens in the irradiated region. This swelling effect is illustrated in FIG. 1.
In FIG. 1A, free monomers (denoted M) are present in a silicone polymer matrix 10. In FIG. 1B, a mask 20 is used to expose only a portion 30 of the lens to UV radiation. The monomers in the region exposed to the UV radiation undergo polymerization, forming polymers P and slightly changing the refractive index. Over time, as seen in FIG. 1C, monomers from the un-exposed regions 40, 50 then migrate into the exposed region 30, causing that region to swell. This change in the lens thickness then leads to a larger change in the optical power. In FIG. 1D, after the migration of the monomer is finished, the whole lens is then exposed to UV radiation to freeze the changes.
There are several shortcomings to this method. One is that the primary change in the lens optical power is due to diffusion of monomer, which is a relatively slow process. Another shortcoming is that the dependence on diffusion as the operative effect limits the spatial resolution of the changes in the lens optical power. A third shortcoming is that the increase in lens thickness in the exposed region forces a thickness decrease in adjacent regions, as monomer from the adjacent region diffuses into the exposed region. This change in thickness in the adjacent regions is not easily controllable. Lenses without these shortcomings and others are desirable.